Direct-current amplifier



S. FREEDMAN ET AL Oct. 3, 1950 DIRECT CURRENT AMPLIFIER 2 Sheets-Sheet 1Filed Sept. 18, 1945 Oct. 3, 1950 s. FREEDMAN v15T AL DIRECT CURRENTAMPLIFIER 2 Sheets-Sheet 2 Filed Sept. 18, 1945 uvm.

.ATTORNEY Patented Oct. 3, 1950 DIRECT-CURRENT AMPLIFIER SamuelFreedman, United States Navy, and Giusto Fonda Bonardi, Manhattan Beach,

Calif.

Application September 18, 1945, Serial No. 617,136

2 Claims. (Cl.`179-171) This invention relates to electronic systems andapparatus for amplification of a steady direct current and slowlyfluctuating direct currents. Y

A principal object of this invention is to provide a method of and anelectronic system and apparatus for amplification, indication, andmeasurement of direct current and slowly liuctuating direct currentswith improved stability and dependability not readily heretoforeattained with conventional direct-current amplifiers.

Previous circuit arrangements and apparatus for this purpose dependedupon direct coupling between the output of one vacuum tube stage and theinput of the next stage, and the direct current or varying voltage undertest was directly applied to the tubes. Such arrangements often resultedin false and erratic indications due to pickup of spurious currents andto the occurrence of small plate or filament voltage fluctuation in thevacuum tube circuits themselves.

Greater stability and dependability, as well as increased voltageamplication, are obtained by means of this invention by incorporatingcertain conventional techniques, such as, for eX- ample, employing theprinciples of intermediatefrequency stages, as used in certain radioreceiving systems, between the various amplifying sections.

More particularly the amplification of direct current is obtained withthe present invention by generating an alternating current by means of asuitable oscillator, and modulating this alternating current with thedirect current, or other signal, where amplification is desired. Theresult is an amplitude-modulated signal with the direct current as itsenvelope of modulation. This signal is then amplied by means of asuitable highgain alternating-current amplifier, detected, andtranslated.

Further objects and advantages of this invention, as well as apparatus,arrangement, operation and the method used, will be apparent from thefollowing description'and claims in connection with the accompanyingdrawings in which:

Fig. 1 is a schematic block diagram of the major components of thesystem involved.

Fig. 2 is a schematic diagram of the complete system, excepting thesources of energy.

In Fig. 1, the major block components are shown in sequence similar tothe actual circuit indicated in Fig. 2. In Fig. l the oscillator |00generates a uniform alternating current of the form indicated at IOI,which current is fed to the modulator |02 and there becomes modulated,for example, by a series of applied direct-current input voltages of theforms shown at |03, |04 and |05. The output of the modulator is of theform indicated by stepped curves |06, |01 and |08. This output is thenfed to the rst amplier |09 of which the output is indicated by curvesIa, |062) and I06c. The latter output now passes through clipper stageIIO and emerges as shown by half-wave curves I I, I I2 and I I3. Thisoutput is then in turn fed into the second amplier I I4 and emerges asindicated by curves IIIa, I |211 and I I3a. The amplified output is thenfed into detector stage H5 where it is rectified and emerges as D. C.indicated at |I6, II'I and H8. Three values of direct-current inputvoltages |03, |04 and I 05 are shown by way of example instead of asingle D. C. voltage.

As shown in Fig. 2, the oscillator I 00 may consist of a vacuum tube II9 of any suitable type for generating oscillations ofV small butconstant amplitude. Any suitable frequency may be employed for thispurpose and the practice followed for interstage coupling by means oftuned circuits commonly used in intermediate or radio frequencyinterstage coupling techniques may be used. The amplitude stability is,however, very important. The circuit of this oscillator includes a tunedgrid circuit |20 in inductive relationship with plate circuit coil I2|arranged to provide the necessary feedback for generating oscillations.The oscillator may, if desired, consist of any other convenient type ofcircuit. The proper amount of negative feedback is injected in thecathode circuit by means of the resistor |22. This negative feedbacktends to keep the amplitude of the oscillation small and to increase theamplitude stability. The oscillator control potentiometer |23 in theplate circuit provides means for adjusting the amplitude of the outputwithout aiecting the stability of the oscillator. Switch |24 permitsswitching the coupling circuit on and off as desired when adjustmentsare being made. The coupling circuit is of the double tuned typeemploying coupling coils |25 and |26.

This coupling should be sufficiently loose so asito prevent the smallload changes, due to adjustments of the circuits that follow, fromaffecting the amplitude of the oscillations. The tuned circuits aresimilar to conventional intermediate frequency circuits and can betuned, trimmed, coupled, and shielded as currently used inintermediate-frequency techniques. The lter section, comprisingcondenser |21 and the resistor |28 in the plate circuit, assists inkeeping constant the amplitude of the oscillation.

The modulator consists of a suitable vacuum tube |29 having a remotecutoff characteristic that permits the variation of its output amplitudeto be proportional to the variations of grid bias. This occurs when thecharacteristic curve closely follows the square law, as then the currentis proportional to the square of signal voltage. The average value ofgrid bias should therefore be kept near that point. This means that forbest results the adjustment of the modulator should be maintained at thepoint of best linearity, although this adjustment may not always be tube|42, and in connection with meter switch |36 in position |45, aids indetermining the proper bias. In this position of the circuit the biaswill be about volts, although the optimum value should be determined byexperimentation.

The detector ||5 consists, as shown in Figure 2, of a biased detectortube |41, such as a pentode the same as that required for suitableamplica- Y tion of the small voltage fluctuations; the amplifier controlpotentiometer |3|, which may cover all values of D. C. input signalsfrom 0 to 100 volts, is therefore provided. A resistor |32 in the inputD. C. circuit provides a D. C. connection for the grid during thepreliminary adjustment when no D. C. signal is present at the inputterminals |33 and |34. The resistor |35 in the plate circuit of the tube|29, in connection with the lever of switch |36 in position |31, makesit possible to adjust the amplifier control potentiometer |3| until theoperating point of the tube |29 falls in the square law position of itscharacteristic curve. This particular portion of the circuit is adjusteduntil the voltmeter |38 reads 50 volts indicating that the right currentcorresponding to the proper bias is flowing through tube |29. This tube,in addition to functioning as a modulator, is also the first amplifier(indicated as |09 in Figure 1) being connected to the tuned outputcoupling coil |39. In the voltage regulating position of the circuit twovoltage regulating vacuum tubes |40 and |4|, of any suitable type,provide good regulation of the cathode voltage against the ampliercontrol potentiometer |3| as well as good regulation of the screenvoltage of tube |29. The resistor |30, in series with the voltageregulating tubes |48 and 4| in the B source positive supply line, is theballast resistor. This voltage regulating section is also connected withthe stage that follows it to provide a similar control of the cathodeand screen voltage of the clipper tube |42.

kSince the best operating point of the modulator tube |29, with respectto linearity, does not necessarily correspond with the best operatingpoint with respect to the gain of small fluctuations, a clipper tube |42is used to eliminate everything except the abnormal amplitude uctuationswhich represent the desired signal. The use of a sharp cutoff pentodetube |42, which may be of a 6SJ7 or other suitable type, provides asharp clipping of the input oscillations if the grid bias is adjustedbeyond plate cutoff. In this case only a part of the positive peak ofeach oscillation allows plate current to flow. The tuned plate circuit|43 functions as a class C amplier, thereby permitting tube |42 tofunction as the second amplifier as indicated by ||4 in the blockdiagram of Figure 1. The grid bias is adjusted by means of a clippingcontrol potentiometer |44. The resistor |45, in the plate circuit oftube of the GSJ'Y or other suitable type, to detect and cause theamplied D. C. signal to be indicated on the meter |38 when switch lever|36 is connected to position number |48. Detector control potentiometer|49 is used to adjust the current to zero with zero signal; thiscondition is indicated on the meter |38 when switch lever |38 is turnedto contact |48 in the Reading position. VIf greater sensitivity isdesired the meter |38 may be included in a bridge circuit or inconnection with other suitable modifications thereof, since a bridgecircuit is more sensitive than a simple meter connection.

This direct current amplier as described up to the last stage, but notincluding the detector circuit, may also be used in connection with thecontrolled voltage divider circuit described in copending application,Serial Number 608,243, iiled August 1, 1945, in order to utilize theadvantages of the controlled Voltage divider with small D. C. signals.In this case detection is provided by the controlled voltage dividersystem itself. The meter |38 may be calibrated by checking with knownsignals in the event an actual value is desired instead of merely anindication of change in D. C. voltage. This may be accomplished byfeeding a signal of known value to the D. C. input terminals |33 and |34of the amplier |29 and adjusting the meter control potentiometer |50.

Alignment instructions (a) Move switch |36 to the Set to 50 position|31.

(o) Move switch |24 to the On position |5|.

(c) Move lever |5| of oscillator control potentiometer |23 to theextreme right and then move it slightly back to the left.

(d) Adjust amplifier control potentiometer |3| until meter |38 reads 50.

(e) Move switch lever |24 to the Off position |52.

(j) Move switch lever |36 to the Set to 10 position |46.

(g) Adjust the clipping control potentiometer |44 until meter |38 readsZero (Caution: Do not go below Zero; stop when the meter reaches Zero.)

(h) Move switch lever |24 to the On position.

(i) Adjust lever |5| of oscillator control potentiorneter |23 untilmeter |38 reads 10.

(j) Move switch lever |36 to the Reading position |48.

(lc) Move switch lever |24 to Ofi position |52.

(l) Adjust detector control potentiometer |49 until meter |38 reads Zero(Use same caution as for (o) above.)

(m) Move switch lever |24 to the On position.

The D. C. amplifier should now be ready to function. Operations (a) to(m) should be made before the amplifier is used as they represent thebasic alignment of the amplifier. They need not be repeated every time,and they should be made with no D. C. signal' in the input terminals |33and |34. The potentiometers involved in operations (g) (if) and (l)should not again be touched except when checking the entire alignmentperi- Odially by repeating all of the above operations.

perating instructions l (ce) Move switch lever las te the set te oposition |31.

(bb) Move the amplifier control potentiometer reading on meter |38,while any decrease in the signal will decrease the reading on the metera proportional amount. l

The meter |38 may be calibrated by checking with input signals of knownstrength in the event an actual measure of the signal is requiredinstead of only an indication of its D. C. voltage change.

The maximum D. C. signal input voltage of the apparatus as indicated is100 volts. The meter |38 may be a volt meter with 100 volts full scaledeiiection and having an internal resistance as high as possible.

The values of all apparatus are for example only, and it is to beunderstood that any other suitable apparatus and values may besubstituted as found necessary or desirable.

It will be readily understood that for some purposes the operation ofthis invention may be eiected by omitting the clipper stage H0, andsecond amplifier stage i4, of Figure 1 and the corresponding apparatusin Figure 2.

The title of this application has been selected for identificationpurposes only and it is not to be construed as fully describing thisinvention, or in limitation of its scope and application.

The apparatus and values listed hereinafter are also by way of exampleonly and it is to be understood that other apparatus and other suitablevalues may be substituted as desired or required.

Lzst of parts, values and functzons Symbol Value Functions RESISTORS 1221,000 Ohms (1 Watt) Un-by-passed cathode resistor or" oscillator tube119. 128 3,000 Ohms 1 Watt) Filter resistor in the plate circuit ofoscillator 119. 130 4,000 Ohms (2 watts) Ballast resistor for voltageregulating section. 132 2 Megobms (l/ Watt) Input D. C. connection tomodulator tube 129. 135 10,000 Ohms (2 watts) Plate resistor ofmodulator tube 129. 145 10,000 Ohms (2 Watts) Pl1a4t2e resistor ofclipper tube 153 1,000 Ohms (l watt) By-passed cathode resistor ofoscillator tube 110. 154 5,000 Ohms (4 watts) Screen voltage divider ofdetector tube 147. 155 5,000 Ohms (4 Watts). Cathode voltage divider ofdetector tube 147.

Symbol Value Functions POTENTIOMETERS 123 10,000 Ohms (1 watt)-Oscillator control plate output potentiometer of oscillator Y mbe 119.

131 50,000 Ohms (2 watts) Amplitude control cathode potenltigmeter ofmodulator 142. Detector Control cathode po- 149 5,000 Ohms (4 Watts)lzrlitiometer of detector tube 150 50,000 ohms 2 wenn--. Merel controlpute potentiometer of detector tube 147.

CONDENSERS4 127 .05 mid Filter condenser' in the plate circuit ofoscillator tube 119.

156 .01 mid Cathode by-pass of oscillator tube 119.

157 .1 mfd Output tuned circuit by-pass o f oscillator tube 119.

158 .005 mid by-pass of modulator tube 159Y .05 mfd Plate by-pass ofmodulator tube 129.

160 .1 mid Cathode by-pass of clipper tube 142.

161 Plate-screen by-pass of clipper tube 142.

162 by-pass of detector tube 163 Cathode by-pass of detector tube 147.

164 Sclreen by-pass detector tube 165 Plt? by-pass of detector tubeVACUUM TUBES 119 6SF6 tube Oscillator.

129 6SK7 tube- Modulator and First amplifier.

140 VR105 Voltage regulator for screen grid of tubes 129 and 142.

141 VR105 Voltage regulator for cathodes of tubes 129 and 142.

142 GSJ 7 tube Clipper and Second amplier.

147 GSI? tube Detector.

TUNED CIRCUITS 120 Oscillator-grid tuned circuit. 125 Oscillator outputtuned circuit.

Modulator input tuned circuit. Modulator output tuned circuit. Clipperoutput tuned circuit. Clipper input tuned circuit. Detector input tunedcircuit.

It is to be understood that various modifications and changes may bemade in this invention without departing from the spirit and scopethereof as set forth in the appended claims.

What is claimed is:

l. The combination, in a system for amplifying and measuring directcurrent, of means for generating oscillations, means for modulating saidoscillations by the direct current to be amplified, means for amplifyingsaid modulated oscillations, means for eliminating the negative halfcycle portions of said amplified modulated oscillations, means foramplifying the positive half cycles, and means for rectifying andtranslating the resulting component of said half waves afteramplification.

2. The combination, in a system for amplifying direct current, of avacuum tube oscillator for generating oscillations of substantiallyuniform amplitude, a vacuum tube modulator whereby said oscillations aremodulated by the direct current to be amplified and measured, a vacuumtube amplifier for amplifying said modulated oscillations, means forrectifying the amplified oscillatentiometer of clipper tubeV 2,524,165 78 tions, thereby to provide a varying signal of pre- REFERENCES CITEDdetermined polarity an ampner for amplifying The following referencesare of record in the said polarized varying signal, and Va. vacuum tubefue of this patent: detector for detecting and rectifying the amplifiedversion of said polarized signal. 5 UNITED STATES PATENTS Number NameDate SAMUEL FREEDMAN. 1,428,156 Espenschied Sept. 5, 1922 GIUSTO FONDABONARDI. 2,320,476 Schrader et al June 1, 1943

